Base56 Unique ID Generator

base56.py

# Porting from http://blog.kevburnsjr.com/php-unique-hash
# and modified some codes to support variable length

# Min length of unique id
MIN_LENGTH = 2

# Unique ID contains some of these 56 chars
CHARSET = 'ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnpqrstuvwxyz23456789'

# Some primes greater than 56 ^ n / 1.61803398874989484
PRIMES = (1, 41, 2377, 147299, 9132313, 566201239, 35104476161, 2176477521929,
          134941606358731, 8366379594239857, 518715534842869223)

# Modular multiplicative inverses of the primes above a modulo 56 ^ n
PRIME_MMIS = (0, 41, 1913, 138827, 8332073, 245539879, 30024414209,
              1027858277945, 218295060195, 4497555361599889,
              110048532750208471)


def base56_encode(num):
    encoded = ""
    while num > 0:
        mod = num % 56
        encoded += CHARSET[mod]
        num = num / 56
    return encoded


def base56_decode(encoded):
    num = 0
    i = 0
    for char in encoded:
        dec = CHARSET.index(char)
        num += dec * 56 ** i
        i += 1
    return num


def mmi_hash(num):
    length = 0
    for order in xrange(len(PRIMES)):
        length = order
        if num < 56 ** order:
            break
    if length < MIN_LENGTH:
        length = MIN_LENGTH
    ceil = pow(56, length)
    dec = (num * PRIMES[length]) % ceil
    return base56_encode(dec).zfill(length)


def mmi_unhash(hash):
    ceil = pow(56, len(hash))
    num = base56_decode(hash.strip('0'))
    dec = (num * PRIME_MMIS[len(hash)]) % ceil
    return dec
    
    
# testing
if __name__ == '__main__':
    import random
    rand_num = random.randint(1, 56**10)
    assert rand_num == mmi_unhash(mmi_hash(rand_num))